Typically, positioning receivers utilizing the GPS system attempt to receive the signal of at least four different satellites, on the basis of which the position (x, y and z coordinates) of the positioning receiver as well as the time data of the GPS system are computed in the positioning receiver. The positioning is performed, for example, at the stage when the positioning receiver is turned on. It takes a long time to receive four different satellite signals, because the positioning receiver must perform acquisition of each of the signal to be received. For the acquisition and for receiving the signal from the satellites, a positioning receiver of prior art must find out the number, Doppler frequency and code phase of each signal to be received, before the signal can be received. The number of operations required for this step can be estimated by means of the number of satellites in the system, variations in the Doppler frequency, and the number of different code phases. For example, assuming that the satellite positioning system comprises 32 satellites, Doppler frequencies are searched from 21 different frequency ranges and there are 1023 different code phases, and the resolution of ½ chip is applied, the product of these numerical values will give an idea about the extent of the problem. With the above-presented figures, the number of different combinations for the acquisition of the signal from a single satellite will be in the order of 1,300,000 (=32×21×2046). The above-presented numerical values correspond well to the situation in the GPS system presently in use.
In some cases, the above-presented situation is made easier by the fact that the positioning receiver has previously received the signal of the satellites and has some kind of estimation of its own position as well as satellite orbit data. It is thus possible to use a previously determined position as a default position. When the positioning receiver has some kind of time data, position data and satellite orbit data, it is possible, by calculation, to estimate the azimuth angles of the satellites to the positioning receiver and to use this data to restrict the acquisition of the signals of such satellites only, which are, on the basis of the computation, above the horizon in relation to the position of the positioning receiver at the time. Furthermore, assuming that the positioning receiver stays still or moves slowly in relation to the movement of the satellites, it is possible to further restrict the searching range of the Doppler frequency of the signal propagated along the line of sight.
However, the positioning receiver does not necessarily have precise time data of the GPS system, wherein the positioning receiver will not be capable of estimating the position of the satellites at a very high precision.
However, to restrict the searching range of the code phases, the positioning receiver should have very precise time data, because the length of one chip is very short. For example, in the GPS system, one chip is about 0.98 μs and the code iteration interval (epoch) is about 1 ms. Such precision cannot be achieved with conventional real time clocks (RTC) used in positioning receivers. Therefore, in conventional positioning receivers, the estimation of code phases of such satellite signals which have not been acquired, is not possible until the acquisition of signals from at least four different satellites has been completed and the position of the positioning receiver and the time data of the system have been determined on the basis of these signals. In conventional positioning receivers of the prior art, the acquisition of four signals is performed and the position and the time data are determined without knowledge about the code phase. First after these four different signals have been found and received and used for determining the time data of the system and the position of the positioning receiver at some precision, the signals of other satellites are searched and acquired by using the code phase data. The information to be determined on the basis of these satellites can be used to improve the positioning precision. This acquisition of four different satellite signals will take a long time, particularly if the signals are weak. In practice, the time taken for the acquisition may thus be several tens of minutes, even several hours, which, in most applications in practice, means that the positioning receiver cannot be used. Particularly indoors, the signal strength may be so weak that acquisition will not be possible and the positioning will not be successful.
The GPS satellite system applies two kinds of satellite orbit data, Ephemeris and Almanac, of which Ephemeris is the more accurate one. The accuracy of the Ephemeris orbit data is less than one metre, whereas the Almanac orbit data will indicate the satellite position at an accuracy of about 0 to 3 km. Each satellite only transmits the Ephemeris orbit data of its own, but the Almanac orbit data is transmitted to all the satellites.